This invention relates to an alarm system.
Alarm systems are known which rely on a blower to increase or decrease the pressure inside the premises to be protected by blowing air into or out of the premises. A pressure sensitive switch detects the change in pressure which results from a door or window being opened and activates the alarm. Such alarms have never been a great success commercially as the sensitivity of the switch results in false alarms. For example, a sudden gust of wind across or against one wall of the premises can change the pressure sufficiently to activate the alarm. Many attempts have been made to overcome this problem and, in one known form of alarm system, a cyclically varying pressure is created in the premises. This can be done by cyclically reversing the motor of the blower or by means of a moving door or paddle. The varying pressure is used to create a modulated electrical signal. Deviations of the modulated signal from its norm are used to activate the alarm. Applicant is not aware of this system having been used commercially and attributes this to its complexity.
In another known system a solid-state thermal flowmeter detects changes in flow rate through a tube connecting the premises to be protected to the atmosphere. The sensitivity required of the flow meter to enable it to detect that the premises have been opened renders the system prone to giving false alarms due to changes in prevailing conditions which have nothing to do with entry into the premises e.g. temperature changes, wind gusts etc.